The present invention relates to a signal processing system using a digital technique, and in particular, relates to a tone signal detecting system in a telephone switching system using a digital processing system.
A prior tone signal detecting system in a telephone switching system is based upon an analog circuit, and has many analog filters, and analog amplifiers. Accordingly, a prior tone signal detecting system is large in size, and since a multiplexed apparatus is impossible, the system has been uneconomical. Further, the regular maintenance is periodically necessary to keep the desirable characteristics of the apparatus, since an analog apparatus is deteriorated by the secular variation.
A prior digital technique for detecting a tone signal in a telephone switching system is a Discrete Fourier Transform (DFT), which samples the input signal in every predetermined period T (second) and provides the sampled values x.sub.0, x.sub.1, x.sub.2, x.sub.3 . . . . The convolution operation is carried out between said sampled values and the sampled values of the reference signals having the frequency f (Hz), which is the same as the frequency to be detected. Thus, two series of products (s.sub.i =x.sub.i .times.sin 2.pi.fT.sub.i), and (c.sub.i =x.sub.i .times.cos 2.pi.fT.sub.i) are obtained. Those series are accumulated in a predetermined duration, respectively. Each of the accumulated values are squared, respectively, and the two squared values are added to each other. The sum of the addition is the spectrum component of the frequency f Hz of the input signal. If the input signal includes the frequency component other than the frequency f Hz, that component is deleted in the above calculation process. The DFT system is suitable for implementing by digital components since only the discrete sampled values of the input signal are utilized. In the case of the digital type exchange system, in particular, where the channel switch comprises the digital elements, because switching is done after all the telephone signals have been converted to digital code such as PCM, Discrete Fourier Transform (DFT) has good prospects of extensive use in the multi-frequency signal receiver installed in this type of exchange system.
FIG. 1 shows an embodiment of a conventional signal detection system using the above mentioned theorem. In FIG. 1, 1-6, representing f.sub.1 -f.sub.6 respectively, form the DFT circuit that processes calculation of frequency components based on DFT. Initially, the input signal is to be input in the form of PCM code through the input terminal 8. This input signal is multiplied by the sample sequences, sin 2.pi.f.sub.1 t and cos 2.pi.f.sub.1 t, by the internal block 1A of the DFT circuit 1. The block 1A comprises 2 multiplicators (circuit) and the code generator (or oscillator) of the sin sequence and cos sequence. The next block 1B comprises 2 adders and the memory for cumulative calculation that cumulatively calculate the results of the aforementioned process.
After the cumulative calculation has been made for a given number of periods, the result is transmitted to the block 1C where each one is squared and then both are added. The DFT circuit 2 also has an equivalent configuration to the above but the frequencies of sin and cos are established at f.sub.2 (Hz). Similarly, by establishing a total of 6 DFT circuits by altering the frequency, each component of the 6 different frequencies may be obtained. The outputs of these DFT circuits are input into the decision circuit 7, which determines whether each frequency component has the prescribed level or not. From the result of this determining process, rationality of the combination of two different frequencies is examined. Thereafter, the numerical information embodied in that combination is obtained and is applied to the output terminal 9. When used as a telephone tone signal receiver, this final numerical information is transferred to the exchange control device of the telephone exchange system.
During the aforementioned conventional convolution operation, the blocks 1A and 1B must complete multiplication and addition within the period of the sampling interval (normally 125 .mu.s) of the input PCM signal. The multiplier, in particular, has a complicated circuit configuration with a relatively slow operational speed because of the carry bit propagation, etc. Therefore, if the circuit configuration of the prior art was used employing time division and in order to have the many time-division-multiplexed PCM input signals operate properly, the operation speed of the multiplier in the initial level block 1A causes a bottle neck, and the number of multiplexes would be limited. This was the shortcoming of the prior art. In the prior art, the configuration is complicated with many expensive multipliers resulting in high prices and less reliability.